Technical Field
The present disclosure relates to a method for producing a β-sialon fluorescent material.
Description of the Related Art
Some types of light emitting devices can emit lights of various hues based on the principle of additive color mixing, using combinations of a light source and a fluorescent material which is excited by the light from the light source to emit light of a hue different from the hue of the light source. In particular, light emitting devices combining a light emitting diode (hereinafter, referred to as “LED”) and a fluorescent material are used in backlights of liquid crystal displays, lighting systems and the like. The color reproduction range of a liquid crystal display can be enlarged, or the color rendering properties of a lighting system can be improved by constituting a light emitting device using a plurality of fluorescent materials, for example, fluorescent materials emitting green lights in combination with fluorescent materials emitting red lights.
As such a fluorescent material, for example, a fluorescent material including a sialon, a solid solution of silicon nitride, has been proposed and, as such a sialon, α-form sialon and β-form sialon having different crystal structures are conventionally known. A fluorescent material including, among these sialons, β-form sialon (hereinafter, also referred to as “β-sialon fluorescent material”) is a green fluorescent material that is excited in a wide range of wavelengths from near ultraviolet light to blue light and emits light having a peak wavelength in the range of 520 nm to 560 nm.
The β-sialon fluorescent material is represented by, for example, the composition formula Si6-zAlzOzN8-z:Eu (0<z≤4.2). The β-sialon fluorescent material is obtained as a calcined product by mixing silicon nitride (Si3N4), aluminum nitride (AlN) and aluminum oxide (Al2O3) together with europium oxide (Eu2O3) as an activator in a prescribed molar ratio and calcining the mixture at about 2000° C. It has been disclosed that β-sialon fluorescent material with high emission intensity can be obtained by heat treating such a calcined product in an inert gas followed by acid treatment (see, for example, JP 2005-255895 A and JP 2011-174015 A). Other approaches that are known to provide higher emission intensity are to perform heat treatment at high temperature in two steps, and to use a β-sialon fluorescent material obtained by calcination as part of the raw materials (see, for example, JP 2007-326981 A and JP 2013-173868 A).